Transient Stabilized SOI FETs

ABSTRACT

Integrated circuits (ICs) that avoid or mitigate creation of changes in accumulated charge in a silicon-on-insulator (SOI) substrate, particularly an SOI substrate having a trap rich layer. In one embodiment, a FET is configured such that, in a standby mode, the FET is turned OFF while maintaining essentially the same VDS as during an active mode. In another embodiment, a FET is configured such that, in a standby mode, current flow through the FET is interrupted while maintaining essentially the same VGS as during the active mode. In another embodiment, a FET is configured such that, in a standby mode, the FET is switched into a very low current state (a “trickle current” state) that keeps both VGS and VDS close to their respective active mode operational voltages. Optionally, S-contacts may be formed in an IC substrate to create protected areas that encompass FETs that are sensitive to accumulated charge effects.

CROSS-REFERENCE TO RELATED APPLICATIONS—CLAIM OF PRIORITY

This application is a continuation of co-pending and commonly assignedU.S. application Ser. No. 16/875,615, filed on May 15, 2020 and“Transient Stabilized SOI FETs”, to issue as U.S. Pat. No. 11,251,140 onFeb. 15, 2022, which is incorporated herein by reference in itsentirety. Application No. 16,875,615 is a divisional of commonlyassigned U.S. application Ser. No. 15/600,579, filed on May 19, 2017 and“Transient Stabilized SOI FETs”, now U.S. Pat. No. 10,672,726 issuedJun. 2, 2020, which is incorporated herein by reference in its entirety.

This application may be related to U.S. patent application Ser. No.15/600,588, filed May 19, 2017, entitled “Managed Substrate Effects ForStabilized SOI FETs”, now U.S. Pat. No. 10,276,371, issued on Apr. 30,2019, Attorney Docket No. PER-232-PAP), to U.S. patent application Ser.No. 14/964,412, filed Dec. 9, 2015, entitled “S-Contact for SOI”, toU.S. patent application Ser. No. 15/385,618, filed Dec. 20, 2016,entitled “Systems, Methods and Apparatus for Enabling High VoltageCircuits”, and to U.S. patent application Ser. No. 15/268,297, filedSep. 16, 2016, entitled “Standby Voltage Condition for Fast RF AmplifierBias Recovery”, all assigned to the assignee of the present inventionand hereby incorporated by reference.

BACKGROUND (1) Technical Field

The invention relates to electronic circuits, and more particularly toradio frequency circuitry fabricated with silicon-on-insulatortechnology.

(2) Background

Modern radio frequency (RF) transceivers have become ubiquitous, withmanifestations found in cellular telephones, wireless networks (e.g.,WiFi, Bluetooth), laptop computers, tablets, computer peripheral devices(e.g., printers, keyboards, mice), etc. That ubiquity has increaseddemand for smaller and lower power RF transceiver circuit designs.

FIG. 1 is a block diagram of a typical prior art transceiver 100 such asthe type that might be used in a cellular telephone. As illustrated, thetransceiver 100 includes a mix of RF analog circuitry for directlyconveying and/or transforming signals on an RF signal path, non-RFanalog circuitry for operational needs outside of the RF signal path(e.g., for bias voltages and switching signals), and digital circuitryfor control and user interface requirements. In this example, a receiverpath Rx includes RF Front End, IF Block, Back-End, and Baseband sections(noting that in some implementations, the lines between sections may beblurred).

The receiver path Rx receives over-the-air RF signals through an antenna102 and a switching unit 104, which may be implemented with activeswitching devices (e.g., field effect transistors or FETs), or withpassive devices that implement frequency-domain multiplexing, such as adiplexer or duplexer. An RF filter 106 passes desired received RFsignals to a low noise amplifier (LNA) 108, the output of which iscombined in a mixer 110 with the output of a first local oscillator 112to produce an intermediate frequency (IF) signal. The IF signal may beamplified by an IF amplifier 114 and subjected to an IF filter 116before being applied to a demodulator 118, which may be coupled to asecond local oscillator 120. The output of the demodulator 118 istransformed to a digital signal by an analog-to-digital converter 122and provided to one or more system components 124 (e.g., a videographics circuit, a sound circuit, memory devices, etc.). The converteddigital signal may represent, for example, images (video or still),sounds, or symbols, such as text or other characters.

In the illustrated example, a transmitter path Tx includes Baseband,Back-End, IF Block, and RF Front End sections (again, in someimplementations, the lines between sections may be blurred). Digitaldata from one or more system components 124 is transformed to an analogsignal by a digital-to-analog converter 126, the output of which isapplied to a modulator 128, which also may be coupled to the secondlocal oscillator 120. The output of the modulator 128 may be subjectedto an IF filter 130 before being amplified by an IF amplifier 132. Theoutput of the IF amplifier 132 is then combined in a mixer 134 with theoutput of the first local oscillator 112 to produce an RF signal. The RFsignal may be amplified by a driver 136, the output of which is appliedto a power amplifier (PA) 138. The amplified RF signal may be coupled toan RF filter 140, the output of which is coupled to the antenna 102through the switching unit 104.

The operation of the transceiver 100 is controlled by a microprocessor142 in known fashion, which interacts with system control components(e.g., user interfaces, memory/storage devices, application programs,operating system software, power control, etc.). In addition, thetransceiver 100 will generally include other circuitry, such as biascircuitry 146 (which may be distributed throughout the transceiver 100in proximity to transistor devices), electro-static discharge (ESD)protection circuits (not shown), testing circuits (not shown), factoryprogramming interfaces (not shown), etc.

As should be apparent to one of ordinary skill in the art, somecomponents of the transceiver 100 may be positioned in a different order(e.g., filters), and some of the components may be implemented asdistinct integrated circuits (ICs). For example, the RF front end may beseparately embodied from the rest of the circuitry (although there is atrend to try to integrate as much of the transceiver functionality aspossible in a single IC). Other components can be (and usually are)added (e.g., additional filters, impedance matching networks, variablephase shifters/attenuators, power dividers, etc.). Importantly, inmodern transceivers, there are often more than one receiver path Rx andtransmitter path Tx—for example, to accommodate multiple frequenciesand/or signaling modalities—so switching and control becomes morecomplex.

A challenge with electronic circuit design in general is that idealcomponents do not exist, particularly when dealing with RF signals. Forexample, the operational characteristics of many passive and activecomponents in an RF signal path are frequency dependent. As anotherexample, a significant problem with RF circuit design is eliminating orcontrolling unwanted cross-effects (“cross-talk”) and self-effects, suchas parasitic capacitances and inductances, undesired signal coupling,performance changes due to environment temperature changes as well asself-heating, and others. The problems of RF design become more acutewhen embodying RF circuits as ICs, where components materials, circuitjuxtaposition, and power constraints add to the difficulties ofoptimizing operational parameters for all components. As one example,FETs are inherently designed to operate with fields, but fields do nothave distinct edges and often cause cross-effects and self-effects. Asanother example, FETs have operating parameters that are subject toprocess, voltage, and temperature (PVT) variations. Accordingly, RFcircuit designs embodied as ICs generally require optimizations of somecircuit parameters at the expense of other parameters.

As an example of such trade-offs, as noted above, a transceiver 100 willgenerally include bias circuitry. FIG. 2 is a block diagram of a priorart bias voltage generation circuit 200. In this example, a stablereference voltage source 202 provides a reference voltage V_(BG). Thereference voltage source 202 may be, for example, a band-gap voltagereference, which is a temperature-independent voltage reference circuitwidely used in ICs that produces an essentially constant voltageregardless of power supply variations, temperature changes, and load.The reference voltage V_(BG) is applied to a voltage-to-currentconverter circuit 204, which essentially divides the reference voltageV_(BG) by a resistance R to generate a reference current I_(REF). Thereference current I_(REF) may be applied to a bias generator circuit 206which generates a specific bias voltage V_(BIAS) suitable for aparticular amplifier 208. The bias generator circuit 206 may be simpleor complex, depending on the needs of the overall circuit; examples of avariety of bias generator circuits are described in U.S. patentapplication Ser. No. 15/268,229, filed Sep. 16, 2016, entitled “CascodeAmplifier Bias Circuits”, assigned to the assignee of the presentinvention and hereby incorporated by reference.

A notable characteristic of RF circuits in general is that differentcomponents may require different optimizations. For example, theamplifier 208 is in the RF signal path and is generally an analogcircuit optimized for performance at RF frequencies. Conversely, whilethe components of the bias voltage generation circuit 200 are alsogenerally analog circuitry, they generally do not operate at RFfrequencies and they may need optimizations that differ from RF signalpath components. A distinct challenge of RF circuit design is thatoptimization for some circuitry may adversely affect optimization ofother circuitry.

In general, for most transceivers, the RF signal path components are themost important circuitry to optimize. It was recognized some time agothat semiconductor-on-insulator (SOI) IC technology is particularlyuseful for such optimization. An important aspect of SOI technology isthat the semiconductor region in which circuitry is formed is isolatedfrom the bulk substrate by an electrically insulating layer. Anadvantage of isolating circuitry from the bulk substrate is a dramaticdecrease in cross-talk and parasitic capacitance, which significantlyimproves speed and power characteristics for RF components.

FIG. 3 is block diagram showing a typical prior art SOI IC structure 300for a single FET. The SOI structure 300 includes a substrate 302, aninsulator layer 304, and an active layer 306. The substrate 302 istypically a semiconductor material such as silicon. The insulator layer304 is a dielectric which is often silicon dioxide formed through theoxidation of a silicon substrate 302; the insulator layer 304 is oftenreferred to as a buried oxide (or “BOX”) layer. The active layer 306 mayinclude some combination of implants and/or layers that include dopants,dielectrics, polysilicon, metal wiring, passivation, and other materialsto form active and/or passive electronic components and/or mechanicalstructures. For example, in the illustrated embodiment, a FET is showncomprising a source S, a drain D, and a gate G atop an insulating layer308. Additional elements, not shown, may include contacts, conductiveinterconnections to other components and/or external connections, andprotective coatings.

While the basic SOI IC structure 300 of FIG. 3 works reasonably well upto a point for RF circuitry such as transceiver components, as frequencyincreases and power specifications are reduced, cross-talk and parasiticelements again begin to affect such parameters as linearity andswitching speed. Accordingly, other optimizations may be required. Forexample, FIG. 4 is block diagram showing an improved prior art SOI ICstructure 400 for a single FET. The SOI structure 400 includes a highresistivity substrate 402, a trap rich layer 404, a BOX insulator layer406, and an active layer 408 that includes a single example FET 410.

The high resistivity of the substrate 402 makes the impedance of theparasitic paths through the substrate 402 higher. Materials used for thehigh resistivity substrate 402 typically include very lightly dopedsilicon such that the high resistivity substrate 402 takes on some ofthe characteristics of an insulator. The use of high resistivitysubstrates alone has proven capable of extending the benefit of SOIstructures for RF communication circuits by roughly two orders offrequency magnitude.

The trap rich layer 404 is a solution to another problem. Although highresistivity substrates alone are capable of reducing substrate loss whenused in SOI IC structures, they are highly susceptible to anotherphenomenon called parasitic surface conduction. The problem of parasiticsurface conduction occurs because the high resistivity substrate 402 iscapable of terminating field lines, but a thin surface region of thehigh resistivity substrate 402 can be formed into an inversion oraccumulation region as charge carriers are affected by changes in signalvoltages in the active layer 408. The degree to which charge carriers inthe thin surface region are displaced is directly altered by signals inthe active layer 408. As a result, without the trap layer 404, thecapacitance of the junction between the high resistivity substrate 402and the active layer 408, as seen by the active layer 408, depends onthe voltage applied, resulting in nonlinearity and a loss of signalpurity. In addition, an applied voltage can invert this interface on theside of the high resistivity substrate 402 and create a channel-likelayer within the thin surface region where charge can move very easilyin a lateral direction despite the fact that the substrate layer 402 ishighly resistive. Therefore, this effect can also lead tosignal-degrading cross-talk in RF communication circuits.

Forming a trap rich layer 404 on top of the substrate layer 402substantially mitigates parasitic surface conduction. The trap richlayer 404 is typically formed as a layer of amorphous or polycrystallinesilicon on a top surface of the substrate 402, and significantlydegrades the mobility of the charge carriers in the thin surface regionof the substrate 402. Since the carriers cannot travel far, theeffective resistance of the substrate 402 is preserved and thecapacitance as seen by the active layer 408 is not as dependent upon thesignals in the active layer 408. The improved RF performance of SOI ICsubstrates with a trap rich layer 404 is so marked that wafers havingthat configuration are commercially available.

Thus, SOI based IC technology improves RF performance, to a point; highresistivity SOI substrates further improve RF performance, to a point;and high resistivity SOI substrates with trap rich layers further stillimproves RF performance. However, the improved performance of SOI-basedRF circuitry formed in the active layer 408 above the trap rich layer404 can adversely affect desired operational characteristics of non-RFcircuitry formed in the active layer 408 above the trap rich layer 404,such as switches and bias circuits.

For example, for some components (e.g., a power amplifier for use in aWiFi transceiver, such as one conforming to the IEEE 802.11ax or802.11ac standards), the component is active for only short bursts oftime (e.g., ˜4 mS in a WiFi power amplifier) and in a low power mode(e.g., “sleep” or “standby” mode) for most of the time to save power. Inthe WiFi example, the standby power consumption for a power amplifiermay be very low (e.g., <10 μA), but the amplifier may be required tohave a very quick sleep-to-active transition time (e.g., <1 μS) and toachieve a very stable gain very soon (e.g., <30 μS) after becomingactive. The low power standby mode may be achieved by essentiallyswitching some of the active circuitry OFF. More generally, manyradio-based systems, particularly battery-operated systems (e.g.,cellular telephones and WiFi radios) make significant use of standbymodes to conserve power usage and/or to preserve battery life.Transitioning from a standby mode to an active mode introduces transienteffects, especially in bias circuits, which can take a long time tosettle out. However, many RF and analog performance requirementstypically do not allow a lot of recovery (“buffer”) time. In general,such radio systems are designed to start transmitting a maximum amountof data as soon as possible. This requires that the radio system wakenfrom standby mode back to active mode and be ready for full performancewithout significant transient effects.

Compounding the challenges of IC design is the fact that an SOI FET candevelop an accumulated charge over time. For example, one type ofaccumulated charge depends upon the potentials at the source S, drain D,and gate G of the FET 410. More particularly, when used in certain SOIcircuit implementations, conventional FETs may operate in an accumulatedcharge regime. This phenomenon is also referred to as a “floating bodyeffect” or “kink effect”, and more generally is the effect of dependenceof the body potential of an SOI FET on the history of its biasing andcarrier recombination processes. The body of the FET forms a capacitorwith respect to the insulated substrate, and accordingly chargeaccumulates on this capacitor. The accumulated charge may cause adverseeffects, such as opening of parasitic transistors in the structure andcausing OFF-state leakages (resulting in higher current consumption),and also causes a history effect, which is the dependence of thethreshold voltage V_(TH) of a FET on the previous states of the FET.

Another aspect of accumulated charge is related to the OFF state of aFET and the resultant effect on device linearity, and is described inU.S. Pat. No. 7,910,993 B2, issued Mar. 22, 1011, entitled “Method andApparatus for use in Improving Linearity of MOSFET'S using anAccumulated Charge Sink” and assigned to the assignee of the presentinvention.

A trap rich layer 404 on an SOI substrate 402 can be a separate sourceof accumulated charge. For example, when FETs in the active layer switchbetween ON and OFF states, the changing electrical fields accompanyingsuch change in state can cause charge to accumulate between the traprich layer 404 and the BOX insulator layer 406. The high impedancenature of the trap rich layer 404 hinders dissipation of that trappedaccumulated charge.

Regardless of source, accumulated charge can take a long time toequilibrate (e.g., >4 mS), and can affect the intrinsic properties of aFET device (e.g., the threshold voltage of the device, V_(TH))—Thus,turning most or all switch FETs OFF to achieve a standby mode powerlevel specification—as in the WiFi example above—results in accumulatedcharge in or near the trap rich layer 404 which takes time to dissipatewhen the FETs are turned back ON to the point of stable gain; that timemay exceed a required sleep-to-active transition specification.

Anything that affects the operating point of a FET (e.g., V_(TH),biasing, current I_(DS), etc.) generally adversely impacts theperformance of circuits based on such devices, such as the gainstability of an amplifier circuit. Because of the accumulated chargephenomenon, SOI FETs essentially have “memory”: that is, changing theoperating point of a FET involves also settling the body of the FET andthe substrate to a new potential. Further, neighboring FETs and chargednodes can affect the intrinsic properties of a FET. Under suchconditions, turning a FET completely OFF and then getting it to turn ONand be completely stable immediately is quite challenging.

Even providing substrate contacts (S-contacts) to mitigate accumulatedcharges of various types, as taught in U.S. patent application Ser. No.14/964,412 referenced above, does not fully mitigate the problem duringoperation (even if the contacts are ohmic), due to the high resistivityof the substrate 402 restricting the flow of charge. Moreover, anythingcoupling to the substrate 402 may change its potential, therebyaffecting nearby FET devices.

Prior attempts to mitigate the accumulated charge problem for FETsfabricated on SOI substrates having a trap rich layer include improvingthe effectiveness of body ties (such as by interspersing ties on thesource side of a FET), using shorter width and/or longer length FETs tolessen accumulated charge (noting that longer length FETs generally donot work for power amplifiers), or adopting process techniques to reducebody-tie resistance. Attempts have also been made at the circuit levelto compensate for the effects of the accumulated charge problem (ratherthan actually mitigate the problem), such as by creating replicacircuits to try to track out gain variation due to accumulated charge.However, such circuit specific solutions are generally inadequate as ageneral solution.

Accordingly, there is a need for methods and circuits that eliminate orreduce the formation of accumulated charge in SOI substrates having atrap rich layer, or which mitigate the effects of accumulated charge onthe performance of FETs fabricated on SOI substrates having a trap richlayer. The present invention addresses these and other needs.

SUMMARY OF THE INVENTION

The invention encompasses several types of radio frequency (RF)integrated circuits (ICs) that avoid or mitigate creation of accumulatedcharge in a silicon-on-insulator (SOI) substrate, particularly an SOIsubstrate having a trap rich layer, by avoiding a fully OFF state for atleast some field effect transistors (FETs) within the circuit. Bykeeping the standby operating conditions of such critical FETs at theactive state or as close to the active state as possible, accumulatedcharge is stabilized at a near-constant level. An important insight intothe functioning of such FETs was the realization that the less thatcertain node voltages of FETs within an SOI IC change, the more stablethe charge that may accumulate as a result of circuit activity in theactive layer of an IC.

In a first general embodiment, a FET is configured such that, in astandby mode, the FET is turned OFF while maintaining essentially thesame V_(DS) as during an active mode, thereby eliminating or reducingchanges in accumulated charge.

In a second general embodiment, a FET is configured such that, in astandby mode, current flow through the FET is interrupted whilemaintaining essentially the same V_(GS) as during the active mode,thereby eliminating or reducing changes in accumulated charge.

In a third general embodiment, a FET is configured such that, in astandby mode, the FET is switched into a very low current state (a“trickle current” state) that keeps both V_(GS) and V_(DS) close totheir respective active mode operational voltages, thereby reducingchanges in accumulated charge.

In some embodiments, S-contacts are formed in an IC substrate to createprotected areas that encompass FETs that are sensitive to accumulatedcharge effects. More specifically, S-contacts substantially surroundeach circuit to be protected, essentially creating corresponding “wells”surrounded at least in part by S-contact “rings”. The rings ofS-contacts reduce substrate impedance and thus settling time of thesubstrate voltage under the circuits, help in shielding the circuitsfrom electrical interference, help in draining accumulated charge fromcertain layers of the IC, and help to improve impedance matching for thecircuits within the wells by preventing uneven substrate potentialbetween circuits.

Notably, embodiments of the invention, particularly the “tricklecurrent” approach described below, help resolve a number of accumulatedcharge effects of SOI substrates (particularly SOI substrates having atrap rich layer), including floating body effects.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a typical prior art transceiver such as thetype that might be used in a cellular telephone.

FIG. 2 is a block diagram of a prior art bias voltage generationcircuit.

FIG. 3 is block diagram showing a typical prior art SOI IC structure fora single FET.

FIG. 4 is block diagram showing an improved prior art SOI IC structurefor a single FET.

FIG. 5A is a schematic diagram of a first embodiment of the presentinvention that stabilizes V_(DS) of a FET when in a standby mode.

FIG. 5B is a schematic of one circuit for generating V_(LOAD) for thecircuit of FIG. 5A.

FIG. 5C is a schematic diagram of the circuit of FIG. 5A configured inan active (non-standby) mode.

FIG. 5D is a schematic diagram of the circuit of FIG. 5A configured in astandby mode.

FIG. 6A is a schematic diagram of a second embodiment of the presentinvention that stabilizes V_(GS) of a FET when in a standby mode.

FIG. 6B is a schematic diagram of the circuit of FIG. 6A, but configuredin a standby mode.

FIG. 7A is a schematic diagram of a third embodiment of the presentinvention that provides a trickle current state for a PFET when in astandby mode.

FIG. 7B is a schematic diagram of a fourth embodiment of the presentinvention that provides a trickle current state for an NFET when in astandby mode.

FIG. 7C is a schematic diagram of a fifth embodiment of the presentinvention that provides a trickle current state for an NFET when in astandby mode, utilizing fixed current sources.

FIG. 7D is a schematic diagram of a fourth embodiment of the presentinvention that provides a trickle current state for a PFET when in astandby mode.

FIG. 8 is a schematic diagram of a current mirror circuit for providinga reference current V_(BG)/R, such as to the bias generator circuit ofFIG. 2.

FIG. 9A is a schematic diagram of a current mirror circuit based on thePFET circuit of FIG. 7A.

FIG. 9B is a schematic diagram of a current mirror circuit based on theNFET circuit of FIG. 7B.

FIG. 10 is a schematic diagram of a current mirror circuit based on thePFET circuit of FIG. 7A having matching drain characteristics.

FIG. 11 is a schematic diagram of a bias circuit based on the NFETcircuit of FIG. 7B used in conjunction with a variable biassource-follower circuit.

FIG. 12 is a schematic diagram of a bias circuit based on the NFETcircuit of FIG. 7C used in conjunction with a current mirror having avariable current source.

FIG. 13 is block diagram showing an SOI IC structure with a trap richlayer, a BOX insulator layer, and substrate contacts for a single FET.

FIG. 14 is a top plan view of an area of a stylized IC that includestwelve example circuits (e.g., current mirrors for bias circuits of apower amplifier).

FIG. 15 is a top plan view of an area of a stylized IC that includestwelve example circuits surrounded by a plurality of S-contacts.

FIG. 16 is a process flow diagram showing a first method for eliminatingor reducing changes in accumulated charge in an integrated circuitsusceptible to accumulated charge and fabricated on asilicon-on-insulator (SOI) substrate.

FIG. 17 is a process flow diagram showing a second method foreliminating or reducing changes in accumulated charge in an integratedcircuit fabricated on an SOI substrate as part of an integrated circuitsusceptible to accumulated charge.

FIG. 18 is a process flow diagram showing a third method for eliminatingor reducing changes in accumulated charge in an integrated circuitsusceptible to accumulated charge and fabricated on an SOI substrate.

FIG. 19 is a process flow diagram showing a fourth method foreliminating or reducing changes in accumulated charge in an integratedcircuit fabricated on an SOI substrate as part of an integrated circuitsusceptible to accumulated charge.

FIG. 20 is a process flow diagram showing a fifth method for eliminatingor reducing changes in accumulated charge in an integrated circuitsusceptible to accumulated charge and fabricated on an SOI substrate.

FIG. 21 is a process flow diagram showing a sixth method for eliminatingor reducing changes in accumulated charge in an integrated circuitfabricated on an SOI substrate as part of an integrated circuitsusceptible to accumulated charge.

FIG. 22 is a process flow diagram showing a seventh method foreliminating or reducing changes in accumulated charge in an integratedcircuit fabricated on an SOI substrate as part of an integrated circuitsusceptible to accumulated charge.

FIG. 23 is a process flow diagram showing an eighth method foreliminating or reducing changes in accumulated charge in an integratedcircuit fabricated on a silicon-on-insulator (SOI) substrate as part ofan integrated circuit susceptible to accumulated charge.

FIG. 24 is a process flow diagram 2400 showing a ninth method foreliminating or reducing changes in accumulated charge in an integratedcircuit susceptible to accumulated charge and fabricated as part of anintegrated circuit on a silicon-on-insulator (SOI) substrate.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

The invention encompasses several types of radio frequency (RF)integrated circuits (ICs) that avoid or mitigate creation of accumulatedcharge in a silicon-on-insulator (SOI) substrate, particularly an SOIsubstrate having a trap rich layer, by avoiding a fully OFF state for atleast some field effect transistors (FETs) within the circuit. Bykeeping the standby operating conditions of such critical FETs at theactive state or as close to the active state as possible, accumulatedcharge is stabilized at a near-constant level. An important insight intothe functioning of such FETs was the realization that the less thatcertain node voltages of FETs within an SOI IC change, the more stablethe charge that may accumulate as a result of circuit activity in theactive layer of an IC.

In specific applications, use of one or more embodiments of the presentinvention lowers standby power consumption of FETs while enabling a veryquick sleep-to-active transition time (e.g., <1 μS) and achieving a verystable gain very soon after becoming active (e.g., <0.05 dB gainstability in <30 μS).

Notably, embodiments of the invention, particularly the “tricklecurrent” approach described below, help resolve a number of accumulatedcharge effects of SOI substrates (particularly SOI substrates having atrap rich layer), including floating body effects.

Fixed V_(DS) Embodiment

FIG. 5A is a schematic diagram of a first embodiment of the presentinvention that stabilizes V_(DS) of a FET when in a standby mode. Shownis a P-type FET M1 coupled between a supply voltage V_(DD) and acontrolled through-path switch S1. The gate of M1 is coupled to either abias voltage V_(BIAS) or to a standby voltage V_(SB) (which may beV_(DD)) through a controlled gate switch S2. As an example, FET M1 maybe a component of a power amplifier bias circuit.

The through-path switch S1 may be switched to connect M1 to either aLoad or a “pseudo” load V_(LOAD). V_(LOAD) may be provided by a voltagesupply that is approximately equal to the voltage present on the drainof M1 during active mode operation. FIG. 5B is a schematic of onecircuit for generating V_(LOAD) for the circuit of FIG. 5A. In thisexample, V_(LOAD) is generated by a simple resistive divider circuitcomprising resistances R1 and R2 series coupled between V_(DD) andcircuit ground. The voltage for V_(LOAD) depends on V_(DD) and the ratioof R1 to R2. Other circuitry may be coupled between M1 and V_(DD) and/orbetween M1 and the Load. An NFET version of the circuit of FIG. 5A wouldlook similar, but “upside down”: S1, Load, and V_(LOAD) would beconnected between V_(DD) and M1, with the source of the NFET version ofM1 coupled to circuit ground; the gate of M1 would be switched betweenV_(BIAS) and circuit ground, and the drain of M1 would be switchedbetween Load and V_(LOAD) by S1.

As should be apparent to one of ordinary skill in the art, S1 and S2 mayeach be implemented as FETs coupled in conventional fashion to functionas a single-pole, double throw (SPDT) switch. The difference between M1and S1/S2 is that M1 is generally modulated by an applied input signal(not shown) and essentially behaves as a variable resistor, while S1 andS2 have two binary states, connecting a common terminal to either afirst or a second node (in FIG. 5A, S1 and S2 are drawn as being in aneutral position but both are generally binary).

FIG. 5C is a schematic diagram of the circuit of FIG. 5A configured inan active (non-standby) mode. In this configuration, the state of S1 isset to couple M1 to the Load and the state of S2 is set to couple thegate of M1 to V_(BIAS). Accordingly, all currents to the gate of M1 andthrough M1 (i.e., source-drain current I_(DS)) flow normally, and M1 isin an active ON mode.

FIG. 5D is a schematic diagram of the circuit of FIG. 5A configured in astandby mode. In this configuration, the state of S1 is set to couple M1to V_(LOAD) and the state of S2 is set to couple the gate of M1 toV_(SB), where V_(SB) is set at a value that will turn all currents OFF.For example, by applying V_(SB)=V_(DD) to the gate of M1, thegate-to-source voltage V_(GS) of M1 becomes 0 V, turning M1 OFF. SinceV_(LOAD) is approximately equal to the voltage present on the drain ofM1 during active mode operation, then V_(DS) across M1 is essentiallythe same as during the active (non-standby) mode, thus reducingoperating point changes to V_(DS). Since V_(DS) of M1 remainsessentially unchanged when shifting from the active mode to the standbymode, little or no additional charge can accumulate that would adverselyaffect the switching characteristics of M1. The result is that M1 can berapidly returned to the active mode by toggling S1 and S2 back to theconfiguration shown in FIG. 5C.

Control signals (not shown) for switches S1 and S2 may be provided, forexample, from a microprocessor 142, or from dedicated power controlcircuitry or external circuitry (for example, if M1 is part of a poweramplifier IC that does not include all of the circuitry for a completetransceiver).

While FIGS. 5A, 5C, and 5D depict a P-type FET, the same principleapplies to an N-type FET: in a standby mode, turn the FET OFF whilemaintaining essentially the same V_(DS) as during the active mode,thereby eliminating or reducing changes in accumulated charge.

In summary, this aspect of the invention encompasses an integratedcircuit susceptible to accumulated charge and fabricated on an SOIsubstrate (particularly an SOI substrate having a trap rich layer),including at least one FET having a V_(DS) characteristic and configuredsuch that, in a standby mode, the FET is turned OFF while maintainingessentially the same V_(DS) characteristic as during an active mode,thereby eliminating or reducing changes in accumulated charge in or nearthe SOI substrate and/or any trap rich layer and/or elsewhere in the FET(e.g., at the gate, drain, or source of the FET).

This aspect of the invention further encompasses a circuit fabricated onan SOI substrate (particularly an SOI substrate having a trap richlayer) as part of an integrated circuit susceptible to accumulatedcharge, the circuit including: at least one FET having a drain, asource, a gate, a V_(DS) characteristic, and a signal path through theFET between the drain and the source; a first switch coupled to the gateof the FET, configured to switchably couple the gate to one of a biasvoltage or a standby voltage source; and a second switch coupled to thesignal path of the FET, configured to switchably couple the signal pathto one of a load or a pseudo-load voltage source; wherein, in an activemode, the first switch couples the gate of the FET to the bias voltageand the second switch couples the signal path of the FET to the load;and wherein, in a standby mode, the first switch couples the gate of theFET to the standby voltage source and the second switch couples thesignal path of the FET to the pseudo-load voltage source, therebymaintaining essentially the same V_(DS) characteristic as during theactive mode, and thereby eliminating or reducing changes in accumulatedcharge in or near the SOI substrate and/or any trap rich layer and/orelsewhere in the FET (e.g., at the gate, drain, or source of the FET).

Fixed V_(GS) Embodiment

FIG. 6A is a schematic diagram of a second embodiment of the presentinvention that stabilizes V_(GS) of a FET when in a standby mode. Shownis a P-type FET M1 coupled between a supply voltage V_(DD) and acontrolled through-path switch S1, which in turn is coupled to a Load.The gate of M1 is coupled to a bias voltage V_(BIAS). S1 may beimplemented as a FET coupled in conventional fashion to function as asingle-pole, single throw (SPST) switch. Accordingly, S1 has two binarystates, a CLOSED state (as shown in FIG. 6A) and an OPEN state (as shownin FIG. 6B). As an example, FET M1 may be a component of a bias circuitfor a power amplifier. Further, other circuitry may be coupled betweenM1 and V_(DD) and/or between M1 and the Load. An NFET version of thecircuit of FIG. 6A would look similar, but “upside down”: S1 and Loadwould be between V_(DD) and the drain of M1, with the source of the NFETversion of M1 coupled to circuit ground, the gate of M1 connected toV_(BIAS), and the drain of M1 coupled through S1 to Load.

In FIG. 6A, M1 is configured in an active (non-standby) mode. In thisconfiguration, the state of S1 is set to CLOSED, thus coupling M1 to theLoad. Accordingly, all currents to the gate of M1 and through M1 (i.e.,source-drain current I_(DS)) flow normally, and M1 is in an active ONmode.

FIG. 6B is a schematic diagram of the circuit of FIG. 6A, but configuredin a standby mode. In this configuration, the state of S1 is set to OPENand hence current cannot flow through M1 to the Load. More particularly,V_(DS) of M1 is 0 V, and V_(GS) of M1 does not change, thus reducingoperating point changes to V_(GS). Since V_(GS) of M1 remainsessentially unchanged when shifting from the active mode to the standbymode, little or no additional charge can accumulate that would adverselyaffect the switching characteristics of M1. The result is that M1 can berapidly returned to the active mode by toggling S1 back to theconfiguration shown in FIG. 6A.

Again, control signals (not shown) for switch S1 may be provided, forexample, from a microprocessor 142, or from dedicated power controlcircuitry or external circuitry. While FIGS. 6A and 6B depict a P-typeFET, the same principle applies to an N-type FET: in a standby mode,interrupt current flow (I_(DS)) through the FET while maintainingessentially the same V_(GS) as during the active mode, therebyeliminating or reducing changes in accumulated charge.

In summary, this aspect of the invention encompasses an integratedcircuit susceptible to accumulated charge and fabricated on an SOIsubstrate (particularly an SOI substrate having a trap rich layer),including at least one FET having a V_(G)s characteristic and configuredsuch that, in a standby mode, current flow through the FET isinterrupted while maintaining essentially the same V_(GS) characteristicas during the active mode, thereby eliminating or reducing changes inaccumulated charge in or near the SOI substrate and/or any trap richlayer and/or elsewhere in the FET (e.g., at the gate, drain, or sourceof the FET).

This aspect of the invention further encompasses a circuit fabricated onan SOI substrate (particularly an SOI substrate having a trap richlayer) as part of an integrated circuit susceptible to accumulatedcharge, the circuit including: at least one FET having a drain, asource, a gate, a V_(GS) characteristic, and a signal path through theFET between the drain and the source; and a switch coupled to the signalpath of the FET, configured to switchably couple the signal path to aload or interrupt current flow through the signal path of the FET;wherein, in an active mode, the switch couples the signal path of theFET to the load; and wherein, in a standby mode, the switch interruptscurrent flow through the signal path of the FET, thereby maintainingessentially the same V_(GS) characteristic as during the active mode,and thereby eliminating or reducing changes in accumulated charge in ornear the SOI substrate and/or any trap rich layer and/or elsewhere inthe FET (e.g., at the gate, drain, or source of the FET).

Trickle Current Embodiment

In the first and second embodiments described above, either V_(GS) orV_(DS) was set to essentially 0 V to turn current flow through a FET OFFor interrupt such current flow. However, to meet a particular powerconsumption specification, a FET does not necessarily have to be shutcompletely OFF or have all current flow interrupted; instead, the FETcan be switched into a very low current state (a “trickle current”state) that keeps both V_(GS) and V_(DS) close to their respectiveactive mode operational voltages. Essentially, trickle currentembodiments of the invention allow enough current to pass through a FETto maintain an active conduction channel in the FET. In contrast, if aFET is shut OFF completely, the conduction channel is no longer formedand needs to be re-formed when bring the FET out of a standby mode.Effectively, a trickle current mode FET circuit passes just barelyenough current to keep the conduction channel active and the FET readyto turn ON when transitioning from the standby mode to the active mode.For example, a trickle current in standby mode of less than 1/1,000 ofthe active mode current will suffice for many applications, while inother applications, a trickle current in standby mode of less than1/10,000 or even 1/100,000 of the active mode current may be moreuseful. As another example, in one power amplifier bias circuit, theactive mode current consumption was about 10 mA, while the standby modecurrent consumption was in the range of about 0.1-1 μA.

FIG. 7A is a schematic diagram of a third embodiment of the presentinvention that provides a trickle current state for a PFET when in astandby mode. Shown is a P-type FET M1 coupled between a supply voltageV_(DD) and a controlled through-path switch S1. The gate of M1 iscoupled to a bias voltage V_(BIAS). Also coupled to the gate of the M1PFET is a feedback voltage 702 from the drain of M1 that providescontrol over the V_(GS) of M1; the feedback voltage 702 may becontrolled or modified in some applications by optional circuitry 704(an example of one such control circuit is shown in FIG. 8). Note thatthe feedback voltage 702 may be V_(BIAS); that is, no external biasvoltage is applied to the gate of M1, just the feedback voltage 702. Asin the above examples, FET M1 may be a component of a power amplifierbias circuit.

The through-path switch S1 may be switched to connect M1 to either anactive mode resistance R_(L) (which may be provided by a Load) or astandby mode resistance R_(SB). As in the embodiments described above,S1 may be implemented as FETs coupled in conventional fashion tofunction as a single-pole, double throw (SPDT) switch such that S1 hastwo binary states, connecting a common terminal to either a first or asecond node (S1 is drawn as being in a neutral position but is generallybinary). Other circuitry may be coupled between M1 and V_(DD) and/orbetween M1 and R_(L).

When the circuit of FIG. 7A is configured in an active (non-standby)mode, the state of S1 is set to couple M1 to R_(L). Accordingly, currentthrough M1 flows normally, and M1 is in an active ON mode. When thecircuit of FIG. 7A is configured in a standby mode, the state of S1 isset to couple M1 to R_(SB). Current through M1 is primarily controlledby the gate of M1. When R_(L) is changed to R_(SB), I_(DS) mustnecessarily change, which happens automatically by the concurrent changeto the V_(GS) of M1 by means of the feedback voltage 702 (if V_(BIAS)was a fixed value and R_(L) was switched to R_(SB), V_(DS) would be near0 V because the normal mode I_(DS)×R_(SB) is much greater than V_(DD)).

The resistance of R_(SB) should be substantially greater than theresistance of R_(L) so as to impede the flow of current through M1 downto a trickle. In general, R_(SB) should have some repeatedly achievableresistance value (taking into account PVT affects) such that the tricklecurrent is above the leakage current of M1. For example, in someembodiments, the ratio of resistances of R_(SB) to R_(L) ranges fromapproximately 100:1 to approximately 1000:1. Accordingly, because asmall amount of trickle current continues to flow through M1 when R_(SB)is switched into circuit, M1 is still active but at much diminishedpower and speed levels.

FIG. 7B is a schematic diagram of a fourth embodiment of the presentinvention that provides a trickle current state for an NFET when in astandby mode. Shown is an N-type FET M1 coupled between circuit groundand a controlled through-path switch S1 (S1 is drawn as being in aneutral position but is generally binary). The gate of M1 is coupled toa bias voltage V_(BIAS). Also coupled to the gate of the M1 NFET is afeedback voltage 702 from the drain of M1 that provides control over theV_(GS) of M1; the feedback voltage 702 may be controlled or modified insome applications by optional circuitry 704. Again note that V_(BIAS)may be entirely provided by the feedback voltage 702.

The through-path switch S1 may be switched to connect M1 to V_(DD)through either an active resistance R_(L) (which may be provided by aLoad) or a standby mode resistance R_(SB). When the circuit of FIG. 7Bis configured in an active (non-standby) mode, the state of S1 is set tocouple M1 to R_(L). Accordingly, current through M1 flows normally, andM1 is in an active ON mode. When the circuit of FIG. 7B is configured ina standby mode, the state of S1 is set to couple M1 to V_(DD) throughR_(SB), and the V_(GS) of M1 is changed as a function of the feedbackvoltage 702, thereby reducing I_(D)S. Again, the resistance of R_(SB)should be substantially greater than the resistance of R_(L) so as toimpede the flow of current through M1 down to a trickle. Accordingly,because a small amount of current continues to flow through M1 whenR_(SB) is switched into circuit, M1 is still active but at muchdiminished power and speed levels.

FIG. 7C is a schematic diagram of a fifth embodiment of the presentinvention that provides a trickle current state for an NFET when in astandby mode, utilizing fixed current sources. Shown is an N-type FET M1coupled between circuit ground and a controlled through-path switch S1(S1 is drawn as being in a neutral position but is generally binary).The gate of M1 is coupled to a bias voltage V_(BIAS). Also coupled tothe gate of the M1 NFET is a feedback voltage 702 from the drain of M1that provides control over the V_(GS) of M1; again, the feedback voltage702 may be controlled or modified in some applications by optionalcircuitry 704, and V_(BIAS) may be entirely provided by the feedbackvoltage 702.

The through-path switch S1 may be switched to connect M1 either to anactive mode current source I_(N) or to a lower power standby modecurrent source I_(SB). When the circuit of FIG. 7C is configured in anactive (non-standby) mode, the state of S1 is set to couple M1 to I_(N).Accordingly, current through M1 flows normally, and M1 is in an activeON mode. When the circuit of FIG. 7C is configured in a standby mode,the state of S1 is set to couple M1 to I_(SB), and the V_(GS) of M1 ischanged as a function of the feedback voltage 702, thereby reducingI_(D)S. The current provided by I_(SB) should be regulated to allow onlya trickle of current to flow through M1. Thus, because a small amount ofcurrent continues to flow through M1 when I_(SB) is switched intocircuit, M1 is still active but at much diminished power and speedlevels.

FIG. 7D is a schematic diagram of a fourth embodiment of the presentinvention that provides a trickle current state for a PFET when in astandby mode. Shown is a P-type FET M1 coupled between a supply voltageV_(DD) and a controlled through-path switch S1. The gate of M1 isselectively coupled by switch S2 to a bias voltage V_(BIAS) or to atrickle voltage V_(TRICKLE). As in the above examples, FET M1 may be acomponent of a power amplifier bias circuit. The embodiments shown inFIGS. 7B and 7C can be similarly configured with a switch S2 to select abias voltage V_(BIAS) or a trickle voltage V_(TRICKLE) rather than use afeedback voltage 702.

The through-path switch S1 may be switched to connect M1 to either anactive mode resistance R_(L) (which may be provided by a Load) or astandby mode resistance R_(SB). Switch S2 would be controlled toconcurrently select between V_(BIAS) (for active mode) or V_(TRICKLE)(for standby mode). As in the embodiments described above, S1 and S2 mayeach be implemented as FETs coupled in conventional fashion to functionas a single-pole, double throw (SPDT) switch such that they have twobinary states, connecting a common terminal to either a first or asecond node (both S1 and S2 are drawn as being in a neutral position butare generally binary). Other circuitry may be coupled between M1 andV_(DD) and/or between M1 and R_(L).

When the circuit of FIG. 7D is configured in an active (non-standby)mode, the state of S1 is set to couple M1 to R_(L) and the state of S2is set to couple V_(BIAS) to the gate of M1. Accordingly, currentthrough M1 flows normally, and M1 is in an active ON mode. When thecircuit of FIG. 7D is configured in a standby mode, the state of S1 isset to couple M1 to R_(SB) and the state of S2 is set to coupleV_(TRICKLE) to the gate of M1, thereby reducing I_(DS).

A common feature of the embodiments of FIGS. 7A-7D is that the switch S1selectively couples the source-drain signal path to one of an activemode normal current flow path, or to a standby mode trickle current pathin which current is either impeded (e.g., by R_(SB)) or regulated (e.g.,by I_(SB)).

In all of the trickle current embodiments described above, since V_(DS)and V_(GS) of M1 change very little when shifting from the active modeto the standby mode, little additional charge can accumulate in or nearthe SOI substrate and/or any trap rich layer and/or elsewhere in the FET(e.g., at the gate, drain, or source of the FET). The result is that M1can be rapidly returned to the active mode by toggling S1 back to theactive mode configuration. In all cases, control signals (not shown) forswitch S1 may be provided, for example, from a microprocessor 142, orfrom dedicated power control circuitry or external circuitry.

Regardless of specific implementation, the same principle applies to alltrickle current embodiments: in a standby mode, substantially restrictcurrent flow (I_(DS)) through a FET while keeping both V_(GS) and V_(DS)close to their respective active mode operational voltages, therebyreducing changes in accumulated charge.

In summary, this aspect of the invention encompasses an integratedcircuit susceptible to accumulated charge and fabricated on an SOIsubstrate (particularly an SOI substrate having a trap rich layer),including at least one FET having a drain, a source, a gate, a V_(DS)characteristic, a V_(GS) characteristic, and a signal path through theFET between the drain and the source, and configured such that, in astandby mode, the FET is switched into a very low current state withrespect to current flow through the signal path that keeps both theV_(GS) characteristic and the V_(DS) characteristic of the FET close torespective active mode operational voltages for the V_(GS)characteristic and the V_(DS) characteristic, thereby reducing changesin accumulated charge in or near SOI substrate and/or any the trap richlayer and/or elsewhere in the FET (e.g., at the gate, drain, or sourceof the FET).

This aspect of the invention further encompasses a circuit fabricated onan SOI substrate (particularly an SOI substrate having a trap richlayer) as part of an integrated circuit susceptible to accumulatedcharge, the circuit including: at least one FET having a drain, asource, a gate, a V_(DS) characteristic, a V_(GS) characteristic, and asignal path through the FET between the drain and the source; and aswitch coupled to the signal path of the FET, configured to switchablycouple the signal path to a normal current flow path or to a tricklecurrent path; wherein, in an active mode, the switch couples the signalpath of the FET to the normal current flow path; and wherein, in astandby mode, the switch couples the signal path of the FET to thetrickle current path such that both the V_(GS) characteristic and theV_(DS) characteristic of the FET are close to respective active modeoperational voltages for the V_(GS) characteristic and the V_(DS)characteristic, thereby reducing changes in accumulated charge in ornear SOI substrate and/or any the trap rich layer and/or elsewhere inthe FET (e.g., at the gate, drain, or source of the FET). The tricklecurrent path may have a high resistance relative to the normal currentflow path, and/or the trickle current path may include a regulatedcurrent source that allows only a trickle of current to flow through theFET relative to the normal current flow path.

Trickle Current Circuit Applications

The trickle current circuits shown in FIGS. 7A-7D can be used in a widevariety of applications. For example, FIG. 8 is a schematic diagram of acurrent mirror circuit 800 for providing a reference current V_(BG)/R,such as to the bias generator circuit 206 of FIG. 2. Based on the PFETcircuit of FIG. 7A, a differential amplifier 802 provides an essentiallyconstant bias voltage V_(BIAS) to M1 and to a current mirror PFET, M2,based on comparing a reference voltage V_(BG) to the voltage at thedrain of M1, V_(SENSE). In both cases, a load (resistance) is switchedto a much higher value in the standby mode.

As in FIG. 2, the reference voltage V_(BG) may be, for example, aband-gap voltage reference. As in the example of FIG. 7A, the currentmirror circuit 800 can be switched between an active mode (with M1coupled to resistance R through switch S1) and a trickle current standbymode (with M1 coupled to higher resistance R_(SB) through switch S1).

FIG. 9A is a schematic diagram of a current mirror circuit 900 based onthe PFET circuit of FIG. 7A. FIG. 9B is a schematic diagram of a currentmirror circuit 920 based on the NFET circuit of FIG. 7B. In eachconfiguration, FET M1 is self-biased to function as a diode and providesa bias voltage to current mirror FET M2, which is typically sized to besignificantly larger than M1 by a scaling factor m; for example, in someembodiments, m=100. In each configuration, FET M3 represents an exampleload, or a replica device for a power amplifier. A usable voltage isprovided at a respective node N, which may be coupled to other circuitrydownstream (not shown); for example, each configuration may be usefulfor biasing a power amplifier FET stack or a low-noise amplifier (LNA)from node N.

In either case, in the low power standby mode (i.e., R_(SB) is switchedinto circuit by S1), normal bias current is replaced with a tricklecurrent. The trickle current reduces the voltage change on each FET gate(for example, compared to pulling a gate to ground) when switchingbetween the active mode and the standby mode. The trickle current alsoreduces voltage changes on FET drains if the normal-mode loads remainconnected. Note that many more current mirror stages may be connected toM2, and each will remain in the preferred low power state as well whenR_(SB) is switched into circuit by S1.

FIG. 10 is a schematic diagram of a current mirror circuit havingmatching drain characteristics. In this example, two SPDT switches, S1 aand S1 b, are configured to switch in unison (but in opposite directionsfor the circuit as drawn; both switches are drawn as being in a neutralposition but are generally binary). PFET M1 is self-biased to functionas a diode. In the normal mode, the drain of PFET M1 is coupled to R0through S1 a, and the drain of current mirror PFET M2 is coupled to aLoad through S1 b. In the standby mode, the drain of M1 is coupled toR_(SB) through S1 a and the drain of M2 is coupled to R_(SB) through S1b, thus shorting the drains together.

Shorting the drains together in the low power standby mode limitsadditional SOI substrate coupling effects because the two FETs act asone device, which means their terminal voltages match and currentdensities are the same (ignoring possible device mismatch). Upon returnto the active mode, the FETs will have the same starting conditions forV_(DS) and V_(GS), which helps active mode current accuracy and settlingtime (note that the active mode configuration may include a feedbackloop, not shown).

In some configurations, separate resistances (e.g., R_(SB)a and R_(SB)b)corresponding to switches S1 a and S1 b may be used to better match thetrickle current requirements of other circuitry (not shown) coupled toM1 and/or M2. As one of ordinary skill in the art would understand, theconcepts illustrated in FIG. 10 are extensible to NFETs.

FIG. 11 is a schematic diagram of a bias circuit based on the NFETcircuit of FIG. 7B used in conjunction with a variable biassource-follower circuit. NFET M1 is self-biased to function as a diodeand provides a bias voltage to NFET MN1, which is typically sized to besignificantly larger than M1 by a scaling factor m. In this example, thedrain of MN1 is controlled by a source-follower NFET MN_(SF) coupled toa Load. Actual implementations may use stacks of two or moresource-follower stages instead of one, as suggested by the dotted lineconnecting the single illustrated NFET MN_(SF) to the Load.

The function of switch S1 is as described above for FIG. 7B, couplingresistance R₀ to M1 in the active mode and coupling much higherresistance R_(SB) to M1 in the standby mode. In the illustrated example,as an added feature, the bias voltage to the gate of MN_(SF) may also bevaried by mode by controlling switch S2 in unison with switch S1 (bothswitches are drawn as being in a neutral position but are generallybinary). In the active mode, switch S2 would be set to supply a normalbias voltage Vb1 to the gate of MN_(SF). However, in the standby mode,switch S2 would be set to supply a lower bias voltage Vb2 to the gate ofMN_(SF). The lower bias voltage Vb2 would keep the drain voltage of MN1closer to where it would be in the active mode (e.g., just above V_(TH)for NFET MN1), thereby reducing changes in accumulated charge.

FIG. 12 is a schematic diagram of a bias circuit based on the NFETcircuit of FIG. 7C used in conjunction with a current mirror having avariable current source. Standby mode current source I_(SB1) and activemode current source I_(N1) can be coupled by a switch S1 a to a diode D.Similarly, standby mode current source I_(SB2) and active mode currentsource I_(N2) can be coupled by a switch S1 b to the sources of PFETS M1and M3. Switches S1 a, S1 b are drawn as being in a neutral position butare generally binary, and both are configured to switch in unison. PFETM1 is biased by the diode D, and is series coupled to NFET M2, which isself-biased to function as a diode and provides a bias voltage to NFETM4. PFET M3 is self-biased to function as a diode and is series coupledto NFET M4. The output ΔV between M3 and M4 may be used to bias poweramplifier circuitry (not shown).

In the normal mode, switches S1 a and S1 b couple normal current sourcesI_(N1) and I_(N2) in-circuit. In the standby mode, switches S1 a and S1b couple low power current sources Ism and I_(SB2) in-circuit. In theillustrated configuration, to prevent large voltage changes on amplifierinput (i.e., large ΔV) when switching between the standby and activemodes, the input to the diode D remains biased with a trickle current inthe low power standby mode, rather than being discharged to ground.

Summary of Embodiments

More generally, embodiments of the invention encompass an integratedcircuit susceptible to accumulated charge and fabricated on an SOIsubstrate, including: at least one FET having a V_(DS) characteristicand a V_(GS) characteristic, and at least one switch coupled to the FETand configured to be set to a standby mode in which no more than atrickle current flows through the at least one FET while maintainingessentially the same V_(DS) characteristic and V_(GS) characteristic forthe at least one FET as during an active mode, thereby eliminating orreducing changes in accumulated charge. Further, one or more of theembodiments described above may be used in the same IC.

Substrate Stabilization

Additional techniques may optionally be used in conjunction withembodiments of the circuits described above. For example, in someembodiments, it may be useful to create protected areas on an SOIsubstrate that encompass FETs that are sensitive to accumulated chargeeffects by surrounding such areas with substrate contacts (S-contacts),such as the type of S-contacts taught in U.S. patent application Ser.No. 14/964,412 referenced above.

An S-contact in the context of an IC structure is a path which providesa resistive conduction path between a contact region at a surface of alayer of the IC structure and a contact region at or near a surface of ahigh resistivity substrate of the IC structure (high resistivityincludes a range of 3,000 to 20,000 or higher ohm-cm; as known to aperson skilled in the art, standard SOI process uses substrates with alow resistivity, typically below 1,000 ohm-cm).

For example, FIG. 13 is block diagram showing an SOI IC structure 1300with a trap rich layer 404, a BOX insulator layer 406, and substratecontacts for a single FET 410. In the illustrated embodiment, which isotherwise similar to FIG. 4, two S-contacts 1302 a, 1302 b penetratethrough corresponding isolation regions 1304 from the active layer 408to or near the upper surface of the high resistivity substrate 402. Thematerial used for the S-contacts 1302 a, 1302 b can be any lowresistivity conductive material, such as polysilicon and various metals(e.g., tungsten, copper, etc.). In the case of an SOI device, theisolation regions 1304 can be shallow trench isolation (STI) regions. Byvirtue of penetrating through the isolation regions 1304 within theactive layer 408, the S-contacts 1302 a, 1302 b remain isolated fromdirect contact with other active regions of the active layer 408. Incommon practice, the S-contacts 1302 a, 1302 b are electricallyconnected, directly or through other circuit elements, to the source Sor the gate G of a FET 410; one possible electrical connection is shownby a dotted line 1306 from the source S of the FET 410 to one S-contact1302 a (other possible contacts are not shown here, but are illustratedin U.S. patent application Ser. No. 14/964,412 referenced above).However, as discussed below, the S-contacts 1302 a, 1302 b mayelectrically connected to circuit ground or to another known potential.

In the case of SOI substrates having a trap rich layer 404, as shown inFIG. 13, an S-contact 1302 a can penetrate through the trap rich layer404 to make direct contact with the high resistivity substrate 402.Alternatively, since the trap rich layer 404 has some conductivity (andmay be as conductive as the high resistivity substrate 402), in someapplications an S-contact 1302 b can make a resistive contact with thehigh resistivity substrate 402 by contacting the surface of the traprich layer 404. In other applications, an S-contact 1302 b can penetratethe trap rich layer 404 to a depth sufficient enough to make a resistivecontact, through a remaining portion of the thickness of the trap richlayer 404, with the high resistivity substrate 402.

In addition to the purposes taught in U.S. patent application Ser. No.14/964,412 referenced above, S-contacts can be used in conjunction withembodiments of the invention (such as the embodiments described above)to create protected areas on an IC substrate that encompass FETs thatare sensitive to accumulated charge effects. For example, FIG. 14 is atop plan view of an area 1400 of a stylized IC that includes twelveexample circuits 1402 of a type susceptible to accumulated chargeresulting from the interaction of an unmodified trap rich layer 404 andtransient changes of state of FETs comprising such circuits (e.g.,current mirrors for bias circuits of a power amplifier). FIG. 15 is atop plan view of an area 1500 of a stylized IC that includes the twelveexample circuits 1402 of FIG. 14 surrounded by a plurality of S-contacts1502. As illustrated, S-contacts 1502 substantially surround eachcircuit 1402, essentially creating corresponding “wells” surrounded byS-contact “rings” (even though not circular). The rings of S-contacts1502 around the wells reduce substrate impedance and thus settling timeof the substrate voltage under the circuits 1402, help in shielding thecircuits 1402 from electrical interference (from each other and fromother circuits outside the area 1500), help in draining accumulatedcharge from the high resistivity substrate 402 and/or trap rich layer404, and help to improve impedance matching for the circuits 1402 withinthe wells by preventing uneven substrate potential between circuits.However, even a single S-contact near a circuit 1402 may provide abenefit.

Each of the S-contacts 1502 may be electrically connected, directly orthrough other circuit elements, to the source S or the gate G of a FET.However, when used with embodiments of the present invention, it may bequite beneficial to connect the S-contacts 1502 to circuit ground or toanother known potential (even the IC supply voltage, V_(DD)), to avoidimposing signals on the S-contacts 1502. Such imposed signals may createaccumulated charge in the high resistivity substrate 402 and/or in ornear the trap layer 404 and/or elsewhere in the FET (e.g., at the gate,drain, or source of the FET), and may arise, e.g., due to varyingvoltages applied to active layer 408 elements, such as the source S orgate G of a FET. While a static potential may be most beneficial in someapplications, in other applications it may be useful to dynamicallychange the potential applied to the S-contacts 1502, such as by raisingor lowering an applied voltage to counteract accumulated charge thatarises during some operational phases (e.g., bursts of signaltransmissions in an active mode versus essentially quiescent periodsduring a standby mode). In some applications, it may be useful topurposefully inject charge into the high resistivity substrate 402and/or the trap layer 404 by biasing the S-contacts 1502 with a suitablevoltage signal. When a potential other than circuit ground is desired,it may be useful use a charge pump or similar means to inject offsettingcharge, or apply a negative potential, or apply a positive potentialthat exceeds the voltage of the IC power supply (e.g., greater thanV_(DD)).

The size, number, and spacing of the S-contacts 1502 generally is amatter of design choice. However, to improve transient effects, wellsdefined by the S-contacts 1502 should be small enough such that thereare essentially no gradients under large circuits 1402 that mightnecessitate additional impedance matching. Accordingly, the size of theS-contact rings should be similar in size to the wells of potentialformed by the S-contacts. Note that complete encirclement of eachcircuit 1402 may not be necessary in all applications, and that apartial ring of S-contacts may suffice. For example, S-contacts may beomitted in some applications for edges of circuits 1402 not shared withother close-by circuits 1402, such as the S-contacts shown within thedotted oval 1506 of FIG. 15. Moreover, while individual “island” typeS-contacts 1502 are illustrated in FIG. 15, S-contacts can be formed astrenches, in known fashion.

If the S-contacts 1502 are biased in some manner, it may be useful toform a guard ring 1508 of S-contacts around the area 1500 to protectother circuitry; S-contact trenches would work particularly well forsuch a guard ring 1508, which typically would be grounded.

In addition to the methods taught in U.S. patent application Ser. No.14/964,412 referenced above, a person skilled in the art will know ofmany fabrication methods to provide S-contacts suitable for the purposesdescribed in this disclosure.

Methods

Another aspect of the invention includes methods for eliminating orreducing changes in accumulated charge in an integrated circuitsusceptible to accumulated charge and fabricated on asilicon-on-insulator (SOI) substrate (particularly an SOI substratehaving a trap rich layer), and for eliminating or reducing changes inaccumulated charge in a circuit fabricated on an SOI substrate(particularly an SOI substrate having a trap rich layer) as part of anintegrated circuit susceptible to accumulated charge. Following are anumber of examples of such methods.

FIG. 16 is a process flow diagram 1600 showing a first method foreliminating or reducing changes in accumulated charge in an integratedcircuit susceptible to accumulated charge and fabricated on asilicon-on-insulator (SOI) substrate, including: providing at least onefield effect transistor (FET) having a V_(DS) characteristic (STEP1602); and configuring the at least one FET such that, in a standbymode, the FET is turned OFF while maintaining essentially the sameV_(DS) characteristic as during an active mode, thereby eliminating orreducing changes in accumulated charge (STEP 1604).

FIG. 17 is a process flow diagram 1700 showing a second method foreliminating or reducing changes in accumulated charge in an integratedcircuit fabricated on an SOI substrate as part of an integrated circuitsusceptible to accumulated charge, including: providing at least one FEThaving a drain, a source, a gate, a V_(DS) characteristic, and a signalpath through the FET between the drain and the source (STEP 1702);coupling a first switch to the gate of the FET and configuring the firstswitch to selectively couple the gate to one of a bias voltage or astandby voltage source (STEP 1704); coupling a second switch to thesignal path of the FET, and configuring the second switch to selectivelycouple the signal path to one of a load or a pseudo-load voltage source(STEP 1706); in an active mode, setting the first switch to couple thegate of the FET to the bias voltage and setting the second switch tocouple the signal path of the FET to the load (STEP 1708); and in astandby mode, setting the first switch to couple the gate of the FET tothe standby voltage source and setting the second switch to couple thesignal path of the FET to the pseudo-load voltage source, therebymaintaining essentially the same V_(D)S characteristic as during theactive mode, and thereby eliminating or reducing changes in accumulatedcharge (STEP 1710).

FIG. 18 is a process flow diagram 1800 showing a third method foreliminating or reducing changes in accumulated charge in an integratedcircuit susceptible to accumulated charge and fabricated on an SOIsubstrate, including: providing at least one FET having a V_(GS)characteristic (STEP 1802); and configuring the at least one FET suchthat, in a standby mode, current flow through the FET is interruptedwhile maintaining essentially the same V_(GS) characteristic as duringan active mode, thereby eliminating or reducing changes in accumulatedcharge (STEP 1804).

FIG. 19 is a process flow diagram 1900 showing a fourth method foreliminating or reducing changes in accumulated charge in an integratedcircuit fabricated on an SOI substrate as part of an integrated circuitsusceptible to accumulated charge, including: providing at least one FEThaving a drain, a source, a gate, a V_(GS) characteristic, and a signalpath through the FET between the drain and the source (STEP 1902);coupling a switch to the signal path of the FET and configuring theswitch to selectively couple the signal path to a load or interruptcurrent flow through the signal path of the FET (STEP 1904); in anactive mode, setting the switch to couple the signal path of the FET tothe load (STEP 1906); and in a standby mode, setting the switch tointerrupt current flow through the signal path of the FET, therebymaintaining essentially the same V_(GS) characteristic as during theactive mode, and thereby eliminating or reducing changes in accumulatedcharge (STEP 1908).

FIG. 20 is a process flow diagram 2000 showing a fifth method foreliminating or reducing changes in accumulated charge in an integratedcircuit susceptible to accumulated charge and fabricated on an SOIsubstrate, including: providing at least one FET having a drain, asource, a gate, a V_(DS) characteristic, a V_(GS) characteristic, and asignal path through the FET between the drain and the source (STEP2002); and configuring the at least one FET such that, in a standbymode, the FET is switched into a very low current state with respect tocurrent flow through the signal path that keeps both the V_(GS)characteristic and the V_(DS) characteristic of the FET close torespective active mode operational voltages for the V_(GS)characteristic and the V_(DS) characteristic, thereby reducing changesin accumulated charge (STEP 2004).

FIG. 21 is a process flow diagram 2100 showing a sixth method foreliminating or reducing changes in accumulated charge in an integratedcircuit fabricated on an SOI substrate as part of an integrated circuitsusceptible to accumulated charge, including: providing at least one FEThaving a drain, a source, a gate, a V_(DS) characteristic, a V_(GS)characteristic, and a signal path through the FET between the drain andthe source (STEP 2102); coupling a switch to the signal path of the FETand configuring the switch to selectively couple the signal path to anormal current flow path or to a trickle current path (STEP 2104); in anactive mode, setting the switch to couple the signal path of the FET tothe normal current flow path (STEP 2106); and in a standby mode, settingthe switch to couple the signal path of the FET to the trickle currentpath such that both the V_(GS) characteristic and the V_(DS)characteristic of the FET are close to respective active modeoperational voltages for the V_(GS) characteristic and the V_(DS)characteristic, thereby reducing changes in accumulated charge (STEP2108).

FIG. 22 is a process flow diagram 2200 showing a seventh method foreliminating or reducing changes in accumulated charge in an integratedcircuit fabricated on an SOI substrate as part of an integrated circuitsusceptible to accumulated charge, including: providing at least one FEThaving a drain, a source, a gate, a V_(DS) characteristic, a V_(GS)characteristic, and a signal path through the FET between the drain andthe source (STEP 2202); coupling a switch to the signal path of the FET,configured to selectively couple the signal path to a load resistance orto a high resistance (STEP 2204); in an active mode, setting the switchto couple the signal path of the FET to the load resistance (STEP 2206);and in a standby mode, setting the switch to couple the signal path ofthe FET to the high resistance, such that both the V_(GS) characteristicand the V_(DS) characteristic of the FET are close to respective activemode operational voltages for the V_(GS) characteristic and the V_(DS)characteristic, thereby reducing changes in accumulated charge (STEP2208).

FIG. 23 is a process flow diagram 2300 showing an eighth method foreliminating or reducing changes in accumulated charge in an integratedcircuit fabricated on a silicon-on-insulator (SOI) as part of anintegrated circuit substrate susceptible to accumulated charge,including: providing at least one FET having a drain, a source, a gate,a V_(DS) characteristic, a V_(GS) characteristic, and a signal paththrough the FET between the drain and the source (STEP 2302); coupling aswitch coupled to the signal path of the FET, configured to selectivelycouple the signal path to a first current source or to a second currentsource, the second current source providing a lower current than thefirst current source (STEP 2302); in an active mode, setting the switchto couple the signal path of the FET to the first current source (STEP2302); and in a standby mode, setting the switch to couple the signalpath of the FET to the second current source, such that both the V_(GS)characteristic and the V_(DS) characteristic of the FET are close torespective active mode operational voltages for the V_(GS)characteristic and the V_(DS) characteristic, thereby reducing changesin accumulated charge (STEP 2302).

FIG. 24 is a process flow diagram 2400 showing a ninth method foreliminating or reducing changes in accumulated charge in an integratedcircuit susceptible to accumulated charge and fabricated on asilicon-on-insulator (SOI) substrate, including: providing at least onefield effect transistor (FET) having a V_(DS) characteristic and aV_(GS) characteristic (STEP 2402); and configuring the at least one FETsuch that, in a standby mode, no significant current flows through theFET while maintaining essentially the same V_(DS) characteristic andV_(GS) characteristic as during an active mode, thereby eliminating orreducing changes in accumulated charge (STEP 2404).

Other aspects of the above methods may include one or more of thefollowing: the standby mode pseudo-load voltage source outputting avoltage approximately equal to the voltage present on the drain of theFET during active mode operation; the trickle current path having a highresistance relative to the normal current flow path; providing aregulated current source coupled to the trickle current path that allowsonly a trickle of current to flow through the FET relative to the normalcurrent flow path; the high resistance being at least about 100 timesgreater than the load resistance; the SOI substrate including a traprich layer susceptible to accumulated charge in or near such trap richlayer; and/or providing one or more (e.g., at least a partial ring)substrate contacts (S-contacts) near or around at least one FET.

Fabrication Technologies and Options

The term “MOSFET”, as used in this disclosure, means any field effecttransistor (FET) with an insulated gate and comprising a metal ormetal-like, insulator, and semiconductor structure. The terms “metal” or“metal-like” include at least one electrically conductive material (suchas aluminum, copper, or other metal, or highly doped polysilicon,graphene, or other electrical conductor), “insulator” includes at leastone insulating material (such as silicon oxide or other dielectricmaterial), and “semiconductor” includes at least one semiconductormaterial.

As should be readily apparent to one of ordinary skill in the art,various embodiments of the invention can be implemented to meet a widevariety of specifications. Unless otherwise

1. An integrated circuit susceptible to accumulated charge andfabricated on a silicon-on-insulator (SOI) substrate, including at leastone field effect transistor (FET) having a V_(GS) characteristic andconfigured such that, in a standby mode, current flow through the FET isinterrupted while maintaining essentially the same V_(GS) characteristicas during an active mode, thereby eliminating or reducing changes inaccumulated charge.
 2. The invention of claim 1, wherein the SOIsubstrate includes a trap rich layer susceptible to accumulated chargein or near such trap rich layer.
 3. The invention of claim 1, furtherincluding at least one substrate contact near at least one FET.
 4. Theinvention of claim 1, further including at least a partial ring ofsubstrate contacts around at least one FET.
 5. A circuit fabricated on asilicon-on-insulator (SOI) substrate as part of an integrated circuitsusceptible to accumulated charge, the circuit including, the circuitincluding: (a) at least one field effect transistor (FET) having adrain, a source, a gate, a V_(GS) characteristic, and a signal paththrough the FET between the drain and the source; and (b) a switchcoupled to the signal path of the FET, configured to selectively couplethe signal path to a load or interrupt current flow through the signalpath of the FET; wherein, in an active mode, the switch couples thesignal path of the FET to the load; and wherein, in a standby mode, theswitch interrupts current flow through the signal path of the FET,thereby maintaining essentially the same V_(GS) characteristic as duringthe active mode, and thereby eliminating or reducing changes inaccumulated charge.
 6. The invention of claim 5, wherein the SOIsubstrate includes a trap rich layer susceptible to accumulated chargein or near such trap rich layer.
 7. The invention of claim 5, furtherincluding at least one substrate contact near at least one FET.
 8. Theinvention of claim 5, further including at least a partial ring ofsubstrate contacts around at least one FET.
 9. A circuit fabricated on asilicon-on-insulator (SOI) substrate as part of an integrated circuitsusceptible to accumulated charge, the circuit including: (a) at leastone field effect transistor (FET) having a drain, a source, a gate, aV_(DS) characteristic, a V_(GS) characteristic, and a signal paththrough the FET between the drain and the source; and (b) a switchcoupled to the signal path of the FET, configured to selectively couplethe signal path to a first current source or to a second current source,the second current source providing a lower current than the firstcurrent source; wherein, in an active mode, the switch couples thesignal path of the FET to the first current source; and wherein, in astandby mode, the switch couples the signal path of the FET to thesecond current source, such that both the V_(GS) characteristic and theV_(DS) characteristic of the FET are close to respective active modeoperational voltages for the V_(GS) characteristic and the V_(DS)characteristic, thereby reducing changes in accumulated charge.
 10. Theinvention of claim 9, wherein the SOI substrate includes a trap richlayer susceptible to accumulated charge in or near such trap rich layer.11. The invention of claim 9, further including at least one substratecontact near at least one FET.
 12. The invention of claim 9, furtherincluding at least a partial ring of substrate contacts around at leastone FET. 13.-33. (canceled)